1. Field of the Invention
The invention relates to a control apparatus and a control method for a variable valve operating mechanism provided in a multi-cylinder internal combustion engine in which a reduced-cylinder operation can be performed.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 5-163971 (JP-A-5-163971) describes a multi-cylinder internal combustion engine that includes a variable valve operating mechanism and a valve stop mechanism. The variable valve operating mechanism includes a variable mechanism portion that changes the valve characteristic of an engine valve, for example, an intake valve and/or an exhaust valve, and an actuator that drives the variable mechanism portion. The valve stop mechanism stops the opening and closing of the engine valve in at least one of cylinders.
In the internal combustion engine, an output from the engine is improved, and properties of exhaust gas are improved by changing the valve characteristic of the engine valve using the variable valve operating mechanism. Also, for example, fuel efficiency is improved by performing a so-called reduced-cylinder operation. In the reduced-cylinder operation, the opening and closing of the engine valve (for example, the intake valve) in at least one of the cylinders is stopped using the valve stop mechanism, and thus, the at least one cylinder is deactivated.
When the actuator of the variable valve operating mechanism is controlled so that an actual value of the valve characteristic matches a target value of the valve characteristic set based on an engine operating state, control characteristic values are set. The control characteristic values are used to set a controlled variable for the actuator, and used to make the actual value of the valve characteristic match the target value of the valve characteristic.
The reaction force of a valve spring of the engine valve is transmitted to the variable valve operating mechanism through a cam of a camshaft. Thus, the reaction force of the valve spring transmitted to the variable valve operating mechanism in the above-described manner (hereinafter, referred to as “cam torque”) influences the optimization of the control characteristic values.
For example, when the feedback control of the controlled variable for the actuator is executed according to the difference between the target value of the valve characteristic and the actual value of the valve characteristic, a change amount in the controlled variable for the actuator is calculated by multiplying the difference by a control gain, which is one of the control characteristic values. The change amount is an amount by which the controlled variable needs to be changed with respect to the controlled variable before the valve characteristic is changed. If the control gain is extremely small with respect to the cam torque, a drive speed, at which the actuator is driven, is decreased, and a response speed is decreased when the valve characteristic is changed. If the control gain is extremely large, the drive speed, at which the actuator is driven, is extremely high, and the actual value of the valve characteristic may overshoot the target value, or a hunting phenomenon, in which the actual value of the valve characteristic oscillates, may occur.
When the actual value of the valve characteristic is maintained at the target value, the controlled variable for the actuator is set to a maintenance value, which is one of the control characteristic values, and at which the actual value is maintained at the target value against the cam torque. If the maintenance value is extremely small with respect to the cam torque, the actual value of the valve characteristic is deviated from the target value in a direction in which the cam torque acts. Therefore, the actuator is driven to correct the deviation. As a result, the hunting phenomenon occurs, that is, the actual value of the valve characteristic oscillates near the target value. When the maintenance value is extremely large, the actual value of the valve characteristic is deviated from the target value in a direction opposite to the direction in which the cam torque acts. In this case as well, the actuator is driven to correct the deviation. As a result, the hunting phenomenon occurs, that is, the actual value of the valve characteristic oscillates near the target value.
When controlling the actuator that drives the variable valve operating mechanism, if the control characteristic values are not appropriately set, the response speed becomes excessively high or insufficient, or the actual value overshoots the target value or the hunting phenomenon, in which the actual value oscillates, occurs. This deteriorates performance of controlling the valve characteristic.
As described above, in the internal combustion engine in which at least one of the cylinders can be deactivated by stopping the opening and closing of the engine valve in the at least one cylinder, that is, in the internal combustion engine in which the so-called reduced-cylinder operation can be performed, when an all-cylinder operation is performed, all the engine valves are opened and closed, and thus all the valve springs generate the reaction force. On the other hand, when the reduced-cylinder operation is performed, the opening and closing of at least one engine valve is stopped, and thus, the reaction force generated by the valve springs is decreased according to the number of the engine valves whose opening and closing are stopped. Thus, the cam torque is changed when the operation of the engine is changed between the all-cylinder operation and the reduced-cylinder operation. Therefore, the control characteristic values need to be changed according to the change in the cam torque. However, in a control apparatus described in the above-described publication, the need of setting the control characteristic values according to the change in the cam torque is not taken into account. Therefore, when controlling the actuator of the variable valve mechanism provided in the multi-cylinder internal combustion engine in which the reduced-cylinder operation can be performed, further improvement needs to be made.